A 41-year-old man with HIV is evaluated in a clinic with a 7-day history of rectal pain, rectal bleeding, and constipation. He recently had condomless receptive anal sex. Anoscopic examination shows a mucopurulent discharge and a few shallow mucosal ulcers. Testing of the rectal region is negative for herpes simplex virus, positive for Chlamydia trachomatis, and negative for Neisseria gonorrhoeae. Serologic testing for syphilis is negative.